The large racks of computing and related hardware equipment that are often found in large data centers present a particularly difficult cooling challenge. Most equipment of this type is cooled using forced-air techniques that are aimed at moving cold air from a cold-air source to the hot components inside the racks. One common technique involves delivering cold air through perforated tiles in a raised floor on which the racks sit.
FIG. 1 shows how this forced-air cooling technique is designed to work in an ideal setting with a conventional hardware cabinet 100. Air flowing through one or more perforated tiles 110 in a raised floor 120 rises in front of the hardware cabinet 100 and passes through its front door 130. The front door 130 of the cabinet 100 is perforated to allow penetration of the cold air into the cabinet 100. The cold air distributes itself evenly and fully along the length of the cabinet, so that all of the air flowing through the perforated tile 110 enters the cabinet. Heat-generating electronic systems 1401 . . . X are mounted inside the cabinet up against or very near the front door 120. In the ideal setting, each of the heat-generating systems 1401 . . . X receives the amount of incoming cold air that its own internal air-moving device (e.g., fan) is capable of moving.
Reality, however, very rarely reflects this ideal. In most data centers, chilled air is supplied by one or more air-conditioning (A/C) units with cooling capacities that are determined by the expected total heat load or power consumption of all heat-producing equipment contained within the data center. While the cooling capacities of these A/C units are typically adequate for removing an amount of heat equal to that generated by the equipment in the data center, the amount of chilled air that is actually delivered to the various hardware cabinets varies quite significantly in relation to a number of factors. For example, the flow of chilled air to a particular hardware cabinet depends largely on the under-floor static pressure, which in turn is dependent upon the distance over which the chilled air must travel from the A/C handlers to reach each cabinet. As data centers grow increasingly large and the number of hardware cabinets contained in the typical data center increases rapidly, the physical area covered by the data center grows, which leads to greater travel distances for chilled air delivered by each A/C unit in the data center. Another factor influencing the delivery of chilled air is the amount of air that can flow through the perforated tiles over a given period of time. The placement of the tiles also impacts the flow of chilled air to the hardware cabinets. Quite often, the perforated tiles that were installed when the data center was first populated are not sufficient to handle the current cooling requirements of the data center. Even in cases where a large amount of tile perforation is provided, inadequate under floor static pressure will result in insufficient chilled air delivery.
One condition that frequently results from these and other factors is an insufficient supply of chilled air to hardware cabinets. According to laws of physics, hot air rises and cold air stays low, resulting in air stratification with the hottest air at the top of the cabinet and the coldest air at the bottom. Without chilled air being delivered upwards through the perforated tiles, every heat-generating system inside the cabinet will be exposed to the ambient temperature that prevails at its corresponding elevation. Insufficient chilled air delivery leads to a mixing of hot exhaust air with the incoming chilled air at certain elevations, which hinders and in some cases prevents the cooling of components in the hardware cabinets.
FIG. 2 shows a hardware cabinet 200 that suffers from an insufficient supply of chilled air 210. The heat-generating systems 2401 . . . X in the lower portion of the cabinet 200 pull all of the available chilled air 210 through the perforated door 220 before any of the air has risen the full length of the cabinet. As a result, very little, if any, of the chilled air is available to the heat-generating systems in the upper portion of the cabinet, and these systems begin to take in hot air 230 that has exhausted through and then risen along the back of the cabinet. This recirculation of hot air through the upper portion of the cabinet causes the components in that area to heat even further and, in many cases, to ultimately fail altogether.
The equipment in a data center is typically designed and certified to operate within a specific range of temperatures (e.g., 10° C. to 35° C.). When an insufficient supply of chilled air reaches a hardware cabinet in the data center, a high temperature gradient develops between the top and bottom of the cabinet. This causes the equipment at the top of the rack to reach its upper temperature limit long before the equipment at the bottom of the cabinet does, even though the chilled air delivered through perforated tiles is still well within the specified range. When this occurs, the acceptable range of temperature fluctuation in the chilled air coming through the perforated tiles is much less than it was designed to be.